1. Field of the Invention
This invention relates to an analytical and computational method for estimating the severity of generator unit outage and multi-terminal branch outage contingencies with respect to voltage collapse in large-scale electric power systems. The severity of a contingency is defined by the xe2x80x9cdistance to collapsexe2x80x9d along a given transfer direction. More particularly, this method estimates how much stress, expressed in megawatts (MWs) and/or megavars (MVARs), a power system can handle before a widespread blackout occurs.
2. Description of Prior Art
Voltage collapse is a physical phenomenon found in electric power systems where voltage magnitudes decline rapidly, resulting in widespread disturbances. Voltage collapse has caused widespread blackouts in major metropolitan areas all over the world, for example Tokyo in 1987 and the entire Western System Coordinating Council (WSCC) region including San Francisco and Los Angeles in 1996. Thus, there is a need for a method to determine or estimate a point at which voltage collapse will occur.
U.S. Pat. No. 5,796,628 teaches a method for preventing voltage collapse in a power-generating system in which a performance index is calculated, which is directly correlated to load demands. The performance index is then used to identify weak areas in the power-generating system. A direct relationship between performance index and load demand enables assessment of the status of the power-generating grid system, in order to avoid a potential voltage collapse. Voltage profiles are generated to identify weak areas of the power-generating system, so that certain loads can be shed.
U.S. Pat. No. 5,745,368 teaches a method for analyzing voltage stability of low and high voltage applications in which two or more contingencies of a bulk power supply system are selected, screened and ranked based on a predetermined ranking algorithm. A voltage collapse index is generated and a stable branch of the bulk power supply system is approximated. A voltage versus power curve is created using a plurality of stable equilibrium points. An approximate voltage collapse point is then calculated.
U.S. Pat. No. 5,719,787 teaches on-line dynamic contingency screening of electric power systems. A sequence of contingency classifiers are used in a method for finding the controlling unstable equilibrium point of the power system, known as the boundary of stability region based controlling unstable equilibrium point method (the BCU method). Contingencies identified as definitely stable require no further analysis. Other contingencies classified as being unstable or undecided are applied to a time-domain simulation program to determine if the contingencies are unstable and require further action.
U.S. Pat. No. 4,974,140 teaches a voltage stability discriminating system for a power system in which a multiple load flow solution relating to a voltage stability discrimination is calculated for a power system. A pair of multiple load flow solutions are calculated and are closely located to each other with respect to the voltage stability discrimination for the power system.
U.S. Pat. No. 5,642,000 teaches a method for determining a performance index for power-generating systems, which are directly correlated to load demands. A performance index is used to assess the amount of load increase that a power-generating system can tolerate, prior to voltage collapse conditions. The performance index can also be used to assess whether the system can sustain a contingency without collapse.
U.S. Pat. No. 5,610,834 teaches a method for improving voltage stability security in a power transmission system that has a plurality of buses and a plurality of sources of reactive reserves coupled to the buses. A first voltage enhancement and a second voltage enhancement may include switchable shunt capacitors, synchronous voltage condensers, static var compensators or a combination of such devices. A third enhancement can include a series of capacitors, parallel lines, or a combination of series capacitors and parallel lines.
U.S. Pat. No. 5,594,659 teaches a method for performing voltage stability security assessment for a region of an electric power transmission system in which a multiple contingency analysis is performed for each of a plurality of reactive reserve basins, using single contingencies with a corresponding quantity that exceeds a predetermined threshold.
U.S. Pat. No. 5,566,085 teaches a stability transfer limit calculation for a power network having two or more independent alternating current generators that supply a common load over separate alternating current transmission lines.
Contingency screening is becoming more important in the new deregulated environment. As the electric power industry moves toward an open and competitive electric power market, the commercial success of the new market depends on accurate, up-to-date information. Open Access, one of the basic tenets of the deregulated power industry, allows all parties equal access to the transmission grid. As the number of energy transactions increases, so does the complexity of determining the capability of the transmission network.
To serve all parties equally, the Federal Energy Regulatory Commission (FERC) has mandated that all transmission owners must publicly declare the xe2x80x9cAvailable Transfer Capabilityxe2x80x9d (ATC) of their transmission facilities. ATC has been characterized by the North American Electric Reliability Council (NERC) as a measure of the transfer capability remaining in the physical transmission network for further commercial activity over and above already committed uses. ATC is defined as the Total Transfer Capability (TTC), less the Transmission Reliability Margin (TRM), less the sum of existing transmission commitments (which includes retail customer service) and the Capacity Benefit Margin (CBM).
Currently, the electric power industry has chosen to use a linear approximation technique for determining ATCs. In some cases, xe2x80x9cMW proxiesxe2x80x9d are used to represent voltage and stability problems, but the proxies are only valid for a single operating point yet the power system evolves constantly due to event and loading disturbances. In special situations, a full nonlinear AC power flow simulation is used to back up the linear analysis. Since the nonlinear AC power flow techniques are computationally expensive, the faster linear techniques are used in many situations where nonlinear analysis should be used.
Contingency screening based on linear analysis is not sufficient for determining ATCs, but it is quick. The acceptance of a nonlinear tool would be swift if the tool could screen contingencies faster than it takes to run a full nonlinear AC power flow.
Accordingly, it is one object of this invention to provide a system having the ability to handle the nonlinearity in transfer capability while requiring less than the computation time of a single nonlinear AC power flow solution.
It is another object of this invention to provide a nonlinear system which is capable of screening contingencies faster than it takes to run a full nonlinear AC power flow.
These and other objects of this invention are addressed by a method for estimating the amount of stress, expressed in megawatts and/or megavars, a power system can handle before occurrence of a widespread blackout comprising the steps of estimating a voltage collapse point of said power system following a set of generator unit outages and/or a set of branch outages and calculating a distance to collapse of said power system. Voltage collapse in accordance with the method of this invention is determined by nonlinear contingency screening.
Nonlinear contingency screening is an analytical and computational method for estimating the severity of generator unit outage and multi-terminal branch outage contingencies with respect to voltage collapse in large-scale electric power systems. To determine the severity of generator unit outage and multi-terminal branch outage contingencies with respect to voltage collapse, the nonlinear contingency screening method estimates the (post-contingency) voltage collapse (saddle-node bifurcation) point of a large-scale power system following a generator unit outage or a branch outage. Once the post-contingency state has been estimated, then the xe2x80x9cdistance to collapsexe2x80x9d can be determined. The critical step in determining contingency severity with respect to voltage collapse is the estimation of the post-contingency voltage collapse point of the power system. Nonlinear contingency screening in accordance with the method of this invention estimates the post-contingency voltage collapse point (saddle-node bifurcation point) quickly and accurately.